1. Field of the Invention
This invention relates to an improved process of crystallizing dextrose monohydrate. More particularly, this invention relates to a continuous crystallization process which rapidly forms a lean phase massecuite and a two-stage, continuous-batch process which produces a rich phase massecuite.
2. Description of the Prior Art
It is known and recognized that dextrose can exist in three crystalline forms, the alpha hydrate form, the alpha anhydrous form, and the beta anhydrous form. The alpha anhydrous and beta anhydrous forms are developed from super-saturated solutions of dextrose at elevated temperatures whereas the hydrate form is unique in that it only forms from super-saturated solutions at relatively low temperatures. The temperature at which dextrose hydrate crystallization begins in a hot super-saturated solution of dextrose having the proper moisture content is shown in the Corn Refiner's Association Critical Data Table. Therein, the transition point between hydrate and anhydrous dextrose is given as 50.degree. C. and the percent dextrose is given as 70.91, working with dextrose solutions in pure water. However, in commercial practice, the starch hydrolysates contain small amounts of non-dextrose substances which modify the transition point; therefore, it is preferable to designate a transition range which in this case is from about 50.degree. C. to about 55.degree. C.
The commercial process for producing dextrose hydrate developed by W. B. Newkirk, in the early 1920's, is still in use today. The basic principles of this process are set forth in U.S. Pat. Nos. 1,471,347; 1,508,569; 1,521,830. The object of the Newkirk process was to obtain dextrose hydrate crystals, designated as purgeable crystals, from which the hydrol or mother liquor could be removed by centrifuging and washing.
In commercial practice of the Newkirk process, the dextrose hydrate crystals are produced by contacting a dextrose containing solution or liquor with dextrose hydrate seed crystals under crystallization temperatures in a crystallizer. A massecuite is formed which consists of a mixture of crystals and mother liquor. Twenty to thirty percent of the massecuite from a previous crystallization is used in the form of a mass of seed crystals in each succeeding crystallization. The crystallizer is rapidly filled with dextrose-containing liquor produced by the acid or enzymic hydrolysis of starches. The temperature of the dextrose liquor is from 40.degree. C. to 60.degree. C. The temperature of the mixture of dextrose liquor and crystals from the previous crystallization is 35.degree. C. to 55.degree. C., and the temperature of the massecuite at the completion of crystallization is 20.degree. C. to 35.degree. C. The mixture is kept at the temperature of mixing until part of the dextrose has crystallized and the initially very high super-saturation is reduced. The massecuite is subsequently cooled in the crystallizer to the centrifuging temperature with concomitant further crystallization of dextrose. The conventional crystallization process is accordingly performed as one batch operation, in one vessel, and at a continuously decreasing temperature.
Even with thorough mixing of the dextrose liquor and the mass of crystals, the super-saturation is so high at the beginning of crystallization that the formation of minute new crystals occurs in addition to the continued growth of the crystals already present. These minute new crystals are often produced in such a great number that there is not sufficient D-glucose in solution in the dextrose liquor available to continue the growth of the dextrose crystals to the desired size until the completion of the crystallization. The massecuite thus consists of crystals of widely differing size and is therefore often very difficult to centrifuge. Large crystals facilitate centrifuging because the apparent viscosity of the massecuite containing large crystals is lower than that of a massecuite consisting only of minute crystals or of crystals of widely differing size. The larger the crystals are, the smaller is their surface per unit of weight. Less wash water is required to wash away the mother liquor, which reduces yield losses caused by dissolution of the crystals during centrifuging.
Other disadvantages of the Newkirk process as a commercial practice today are the long periods of time required for crystallization of the rich phase massecuite product. This problem is exacerbated by need to leave 20-25% of crystals in each crystallizer as seed for subsequent crystallizations. The long crystallization times necessitate a large capital investment for equipment and housing. For example, in commercial practice, the average crystallizing time required to form the dextrose hydrate from the dextrose containing liquor is between 40 and 120 hours. The time and labor required for filling and emptying the crystallizers, as well as for maintenance of such equipment, of course, adds more to the cost of operating present commercial systems. Moreover, there is an in-plant sanitation problem which increases the cost still more. It will be readily apparent that there is a long-felt need and desire to provide a more simple and more economical process for obtaining dextrose hydrate.